1. Field of the Invention
The present invention relates to a dimming circuit for adjusting the light output of a lamp by controlling the current flow in the lamp through turning on and off of a switching means. More particularly, the present invention relates to a dimming circuit which puts the switching means to the off-state to protect the circuit, when there flows an excessive current in the switching means in its on-state.
2. Description of the Prior Art
A prior dimming circuit has generally a composition as shown in FIG. 1.
In the figure, a pulse generating circuit 4 outputs a pulse with variable duty ratio, which is applied to the base of a transistor Q.sub.1 for driving the lamp. Further, between the emitter output of the transistor Q.sub.1 and the power source +B, there is connected a lamp 3.
The transistor Q.sub.1 is turned on and off by a pulse from the pulse generating circuit 4, and the lamp 3 radiates light accompanying this on and off operation. The quantity of light radiated by the lamp 3 is adjusted by controlling the current flow in the lamp 3 through variation of the pulse duty by means of a duty varying means that is provided in the pulse generating circuit 4.
In such a prior composition, when the lamp 3 is short-circuited for some reason, the transistor Q.sub.1 undergoes intermittently a state of excessive load, which leads to a problem that the transistor Q.sub.1 may sometimes be broken due to excessive current.